The invention relates to “active implantable medical devices” as defined by Directive 90/385/EEC of 20 Jun. 1990 of the Council of the European Communities. This invention more particularly relates to implants for delivering therapies of stimulation of the vagus nerve, called VNS therapies (Vagus Nerve Stimulation). It more particularly relates to the use of such therapies in patients at risk of heart failure.
Stimulation of the vagus nerve affects cardiovascular functions by reducing the heart rate and the myocardial contractility with decreased length of diastole. Such effects can help reduce the development of cardiac remodeling which may lead to heart failure.
In general, patients with heart failure have a reduced level of physical activity. Often a maximum effort level is correlated with the severity of heart disease. Although physical activity is recognized as a therapy to improve heart function, these patients struggle to produce physical activity due to the reduction in their heart function, so they can only accomplish bits of exercise. Due to the reduction in heart rate and of the myocardial contractility that results from VNS therapy, a VNS therapy may actually be an aggravating factor in exercise tolerance (e.g., because it may reduce the responsiveness of the heart to current activity). Indeed, even in view of physical effort, VNS stimulation may prevents the heart rate from accelerating, neutralizing the beneficial effect of increased physical activity.
To overcome this difficulty it has been suggested, for example, in U.S. 2012/0095530 A1, to adapt the VNS therapy based on the activity level of the patient, which shall be assessed by a suitable sensor such as an accelerometer integrated in the case of the implantable stimulation pulse generator. The sensor signal is compared with a set of predefined thresholds and the VNS stimulation parameters, primarily the stimulation energy, are selectively adjusted according to the crossed thresholds (e.g., to tailor the therapy so that heart rate can increase with the effort be expressed). In this device, the modulation of the VNS therapy is defined in terms of absolute criteria, namely predefined thresholds. However, the level of activity can vary from a patient to another, and also over time for the same patient, depending on the medium and/or long-term evolution (e.g. from one day to another and/or over a period of several days or weeks) of the disease which the patient suffers from. Changes might be an improvement in condition or a worsening.